The present disclosure relates to a media filter having a nonpowered mixing and coagulation basin and a seawater desalination plant using the same, and a nonpowered mixing and coagulation basin and a dissolved air floatation device using the same, and more particularly, to a nonpowered mixing and coagulation basin which generates at least two different turbulent flows inside the mixing and flocculation basin to which raw water so that particles contained in raw water circulate and come into contact with each other by a water current to coagulate into a predetermined size, and a media filter and a dissolved air floatation device having the nonpowered mixing and coagulation basin.
Reverse osmosis out of various sea water desalination methods is a method for obtaining fresh water by applying pressure to seawater to desalt seawater through a reverse osmosis (RO) membrane. The RO membrane is a semipermeable membrane having ultrafine pores with a diameter of about 0.1 nm to 0.5 nm, and has been widely used to desalinate seawater, having the property to selectively penetrate only water molecules but not to penetrate impurities, such as salt.
However, seawater which is raw water contains not only the impurities, such as salt, but also fine suspended matters having coarse particles. Therefore, in order to prevent pollution of the RO membrane by the fine suspended matters, a pre-treatment process for removing the fine suspended matters from seawater is carried out at the front end of various plants for seawater desalination water treatment including the desalination treatment by the RO membrane. Moreover, in order to remove fine suspended matters contained in seawater, a pre-treatment process having a mixing basin, a coagulation basin and a filter basin or a water treatment process having a mixing basin, a coagulation basin and a floatation basin has been generally used.
First, the media filter of the former will be described.
The mixing basin rapidly mixes chemicals and raw water, first coagulates fine suspended matters, and discharges the coagulated fine suspended matters to the coagulation basin. The coagulation basin grows the suspended matters first coagulated by the mixing basin into the size filterable in the filter basin, and then, discharges the suspended matters to the filter basin arranged at the rear end. For this, mechanical agitators are mounted inside the mixing basin and the coagulation basin in order to form a circulating water flow for inducement of coagulation and growth of aggregates through contact between an injected coagulant or an auxiliary coagulant and the suspended matters.
The filter basin filters grown flocculated matters passing a filter media or a membrane made of sand or activated carbon, and especially, sand filtering is a method to remove suspended matters by inducing raw water to the filter basin in which the filter media having a sand layer and a support gravel layer is mounted and passing and passes the filter basin. In order to increase removal efficiency of the suspended matters, dual media filtration (DMF) that has different filter media, such as sand, activated carbon and anthracite, arranged doubly has been used widely.
The dual media filtration is a method to remove suspended matters by passing and filtering suspended matters through a two-layered grain filter media made with sand and anthracite. In general, the dual media filtration has an advantage in that it is easy to secure filter media and it is possible to treat dissolved organic matters because a double-layered filter is provided.
Next, the water treatment system for floatation of the latter will be described.
The mixing basin and the coagulation basin of the water treatment system are generally identical or similar to those of the media filter, but the water treatment system is different from the media filter in that the water treatment system uses not the filter basin but the floatation basin. The floatation basin can collect and remove particles by skimming when the particles rise to the surface. That is, the floatation basin attaches microbubbles to a suspended phase contained in a dispersion medium by a physical action to raise to a limiting surface where the dispersion medium gets in contact with the air in order to induce solid-liquid separation.
Such a flotation basin adopts dissolved air floatation (DAF), induced air flotation (IAF), vacuum flotation, electrolytic flotation, microbiological auto flotation and so on according to microbubble generating methods.
Dissolved air floatation (DAF) is a water treatment method including the steps of: sufficiently dissolving air in water at high pressure; injecting the dissolved air into raw water; forming the air supersaturated in the water, which is decompressed under water, into microbubbles; combining the microbubbles with flocs in the treated raw water; and rapidly raising the bubble-floc combination to the surface of water in order to achieve solid-liquid separation.
It is essential to inject chemicals in order to remove suspended particulate matters during the water treatment process or the seawater desalination process which include all of the former and the latter. In general, because most coagulants used for water treatment form flocs and carries out coagulation as soon as raw water is put in, namely, within one second, it is very important to rapidly mix the coagulant with raw water as soon as possible after the coagulant is put in. The reason is that coagulation efficiency is deteriorated when the coagulant is not rapidly mixed as soon as raw water is put in, namely, when the coagulant is not evenly distributed to the whole raw water. Therefore, core elements for design are injection spots and injection methods when the coagulant is injected, flow forms of raw water and structures of mixing basins.
As the mixing method, there are a mechanical type, a water current type, a diffusion pump type and so on. The mechanical mixing has been used the most widely in the water treatment process. For this, one or more mechanical mixing devices are mounted on a tank or a water channel. General design standards are mixing intensity of 300 sec-1 (G value), mixing period of 10 seconds to 30 seconds, and required power of 2.23 hp to 2.62 hp per 10,000 m3/d. The mechanical flash mixing has several disadvantages in that it is difficult to mix in a moment, in that the mechanical flash mixing causes lots of short circuits, in that it takes too much time and a lot of energy to mix a metal salt coagulant, and in that a rotary shaft and a gear drive frequently get into trouble.
Moreover, because the mixing basin and the coagulation basin respectively have impeller-type mechanical agitators, the mechanical agitators must be continuously operated in order to dispose wastewater. Therefore, the mechanical flash mixing device has additional disadvantage in that it increases facility installation fees and maintenance fees, such as electric charges for operating the mechanical agitators. Furthermore, the mechanical flash mixing device has a further disadvantage in that bad smell is spread when the mechanical agitator is operated because the conventional flash mixing basin has an open structure at the upper part.
Japanese Patent Publication No. 1993-038492 published on Feb. 19, 1993 discloses a method for removing pollutants from seawater supplied to a seawater desalination system, more particularly, a method for removing pollutants by putting an inorganic compound solution and a sulfuric acid solution or a chloride solution into a coagulant and passing the solution through a double-layered filter tower which is filled with sand and anthracite.
Korean Patent No. 0916709 published on Sep. 11, 2009 discloses a nonpowered mixing apparatus within a pipe to maximize mixing efficiency using scattering elements to get in contact with raw water by chemicals which are scattered in the reverse direction to the raw water.
However, these devices still have disadvantages in that facility installation fees and maintenance fees for operating the mechanical agitators are increased because the agitators must be continuously operated to carry out pre-treatment of raw water or in that filtering efficiency is deteriorated because flocs generated by coagulation between chemicals and suspended matters of raw water are not grown enough to be removed in the filter basin when mixing is carried out without power.
Additionally, Korean Patent No. 1133174 published on Apr. 6, 2012 discloses a movable water treatment system and an automatic control method using DAF with an independent power supply for small and medium sized water bodies. Korean Patent No. 1133174 proposes a water treatment system utilizing DAF to a movable vehicle having self-power source which does not need supply of driving power from an external supply source. However, there is still a disadvantage in that a mechanical agitator must be continuously operated for water treatment and it causes equipment installation expenses and maintenance fees for operating the mechanical agitator.